Sticky keys would be more likely than a blown controller. Maybe the board got something to drink at some point. Occasionally the cables develop problems, too. The only way to find out without either a spare board or cable is opening up the board (thin-walled 5.5 mm hex socket), checking for loose rivets and pulling the membrane connections at the controller PCB. There still won't be any easy way of generating input (though someone has mapped out the matrix IIRC, so one could use a piece of wire), but the computer that did the beeping should no longer protest. Then a cleaning and inspection of the membranes and traces and a nuts/bolts mod are in order.
If the problem persists, it could be either the cable or the controller board. Checking the cable for continuity and shorts would only involve a multimeter - nothing fancy needed at all, but one with explicit continuity test (and beep) is much preferred. You need +5V, Gnd, Clock and Data, both continuous and not shorted to anything else.
Oh, and look at the indicator LEDs when you plug the board with adapter in. Ms have a characteristic blinking pattern when powered up, which varies depending on controller. Usually when the controller hangs up early (as on M2s with dead caps), several LEDs remain lit.
